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Frequency domain reverse time migration method is used to reconstruct damages in isotropic and anisotropic plates. Considering multimode overlapping, the Lamb wave signals scattered by the defects may result in artifacts in defect imaging. The scattering signals are thus pre-processed by using a mode separation method based on the vibration symmetry difference between the fundamental guided modes. Based on the multi-element array ultrasonic technique, a numerical study is carried out for defect imaging of aluminum and composite plates by using the frequency reverse time migration method. This paper is organized as follows. Firstly, in order to capture multi-directional information about damages, scattering Lamb wave signals caused by the defects are numerically collected by an annular array of transducers through using the finite element simulation. Secondly, after the pre-processing of mode separation, the separated scattering signals are time-reversed and used to stimulate the corresponding receivers. The Green's function is utilized to back-propagate the scattering Lamb signals in frequency domain, so that the back-propagated acoustic field information of monitored area can be obtained. Finally, the defect images are reconstructed by the cross-correlation between the incident acoustic field and the back-propagated acoustic field. To illustrate the influence of mode separation, the numerical experiments are carried out on an aluminum plate with single defect and on another composite plate with two adjacent identical defects. The reconstructed results from frequency domain reverse time migration method with and without mode separation are compared. The comparison indicates the importance of mode separation. Furthermore, the method is extended to detecting the double adjacent defects with different depths in the composite plate. The imaging result illustrates that the presupposed two adjacent defects with different depths are successfully identified. Numerical results demonstrate that the pre-processing of mode separation helps to effectively remove the artifacts resulting from the multimode interference in the imaging process. The proposed frequency reverse time migration method presents a strong potential for detecting and imaging defects in isotropic and anisotropic plates, which is capable of accurately measuring multi-site defects with information about geometry, size and depth.

In this study, styrene-butadiene-styrene (SBS) modified asphalt binder was aged by thermal oxidation coupled with ultraviolet (UV) radiation. Fluorescence microscopy (FM), scanning electron microscopy (SEM) with energy dispersive x-ray (EDX) detector, surface free energy, optical technology, Fourier transform infrared spectroscopy, and dynamic shear rheometer were used to explore the surface characteristics and trace the chemo-rheological evolution of the asphalt binder at various durations of aging. Both FM and SEM could observe two phases of degradation of the polymer under thermo-oxidation. The surface of UV-aged asphalt binder produced more cracks, observed by FM and SEM, and bulged along these cracks and some fragmented blocks appeared. EDX results indicated that the oxygen content of the surface of the asphalt binder increased with increasing aging duration, leading to more oxygen-containing compounds, more carbonyl and sulfoxide groups, increasing polarity, and decreasing contact angle of water droplets on the surface of the asphalt binder. Coupled aging increased the reflectance in all regions from 200 nm to 2,500 nm, while thermo-oxidative aging alone increased the reflectance from 200 to 400 nm and produced the opposite result in the other region. With increased aging duration, UV-aged asphalt binder becomes brighter and thermo-oxidative aged asphalt binder becomes darker. The asphalt binder samples did not discolor much with varying aging duration, and the color difference was more similar than opposite. The results of testing chemo-rheological properties indicated that the addition of UV radiation significantly increased the stiffness and elasticity and produced more obvious effects on the surface characteristics of variously aged binders.

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